It appears this was never implemented; once the spurious problem message was removed, Inform was translating "[possible spell]" to an I6 noun-filter token, which works only on objects. I have now implemented such tokens using GPRs, and the result appears to work nicely.

Really a different bug fix altogether, but in testing this I was alarmed to find that

Definition: a spell is possible: if it is xyzzy, yes; no.

didn't work, because "it" had the wrong kind of value. I've fixed this as well.